z~7长周期星系团的光谱确认：探测最早的星系和再电离时代
Spectroscopic confirmation of z~7 LBGs: probing the earliest galaxies and the epoch of reionization
摘要：
我们在ESO/VLT上展示了FORS 2超深光谱活动的最终结果，以确认从我们的VLT/Hawk-I成像调查中选择的三个独立领域的z~7“z-波段脱落”候选者。我们特别报道了在NTT深场中新发现的两个红移约为6.7的星系：这两个星系都显示出Ly-alpha发射线，其静止坐标系EW为15-20 A量级，光度为2-4 X 10^{42} erg/s。我们还提出了一个样本的i-dropout星系的超深观测的结果，从中我们设置了一个坚实的上限的分数的闯入者。在观察到的20个z-dropout中，我们确认了5个星系在6.7 < z < 7.1。这系统地低于根据较低红移观测得出的预期：特别是有一个显着缺乏对象与中间Ly-alpha EW（20和55 A之间）。我们的结论是LBG中Ly-alpha发射的分数从z~3到z~6不断增加的趋势很可能从z~6到z~7反转。解释观测到的LAE分数在辍学人口与再电离之间的快速变化需要一个快速演变的中性部分的氢在宇宙中。假设宇宙在z=6时完全电离，并采用Dijkstra et al.（2011）的半解析模型，我们发现我们的数据要求中性氢分数在时间Δ z ~ 1内发生Δ chi_{HI} ~ 0.6量级的变化，前提是逃逸分数在相同的红移间隔内没有显著增加。
英文摘要：
We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO/VLT for the confirmation of z~7 "z--band dropout" candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular we report on two newly discovered galaxies at redshift ~6.7 in the NTT deep field: both galaxies show a Ly-alpha emission line with rest-frame EWs of the order 15-20 A and luminosities of 2-4 X 10^{42} erg/s. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.7 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular there is a significant lack of objects with intermediate Ly-alpha EWs (between 20 and 55 A). We conclude that the trend for the fraction of Ly-alpha emission in LBGs that is constantly increasing from z~3 to z~6 is most probably reversed from z~6 to z~7. Explaining the observed rapid change in the LAE fraction among the drop-out population with reionization requires a fast evolution of the neutral fraction of hydrogen in the Universe. Assuming that the Universe is completely ionized at z=6 and adopting the semi-analytical models of Dijkstra et al. (2011), we find that our data require a change of the neutral hydrogen fraction of the order Delta chi_{HI} ~ 0.6 in a time Delta z ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval.
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